In a communication system defined as Long Term Evolution (LTE) or Evolved Universal Terrestrial Radio Access (E-UTRA) in Third Generation Partnership Project (3GPP) standardization, a function called “Idle mode Signaling Reduction (ISR)” is introduced in order to reduce a frequency of location registration processing that occurs in relationship with a network when a portable terminal device, being compatible with a communication system (such as Global System for Mobile communications (GSM), Universal Mobile Telecommunications System, and the like) in relation to a plurality of RATs, moves among different RATs. When ISR is active, the portable terminal device holds both a location registration information parameter related to GSM or UMTS and a location registration information parameter related to LTE, both the location registration information parameters being received from the network. Then, the portable terminal device becomes ready to move between a GSM/UMTS area and an LTE area, for both of which location registration processing has already been carried out, without location registration processing with the network.
General descriptions on ISR are defined in non-patent literatures NPL1 to NPL3 that are specifications of 3GPP.